1. Field of the Invention
The invention concerns an improved electrostatic spraying apparatus, especially (but not exclusively) for spraying water-based paints.
In this specification and claims, the word "paint" is used in a very broad sense and for convenience: it is intended to cover any material capable of being electrostatically sprayed at an object the surface of which is to be coated, for whatever purpose.
In the ever-widening application of spraying technology it would be highly desirable to be able to use water-based paints. Such paints have very important advantages over other (i.e. non-water-based) paints. Water-based paints may be diluted with ordinary pure water directly before use, in many cases simply with tap water, which means that during transporting and baking or curing of the paints no environmentally harmful vapours arise, as is the case with paints based on organic solvents. A further important advantage of water-based paints is that they are completely nonflammable.
However, the application of water-based paints in electrostatic spraying causes certain difficulties. Paints suitable for electrostatic spraying must satisfy several physical and electrical conditions. Such a condition is, for instance, the electrical resistivity of the paint: it has been shown that such paints may only be electrostatically charged satisfactorily if they possess a semi-conductor characteristic. When the resistivity of the paints is too low, the paints form a short-circuit between the usual charging electrode connected to high tension and the metallic object to be painted. This makes electrostatic paint spraying impossible. Another difficulty in the electrostatic spraying of water-based paints is that if they are not adequately charged they do not form a satisfactory conical spray.
2. Description of the Prior Art
To eliminate these disadvantages proposals have been made according to which atomization is achieved indirectly, and an insulated or earthed paint tank and/or indirect electrical charging is used. Since water-based paints cannot be atomized satisfactorily under the effect of an electrostatic force field alone, atomization has to be promoted by additional means e.g. various mechanical force effects such as centrifugal forces, pneumatic atomization or the like.
When the high-tension electrode makes direct contact with the paint, a significant amount of electrostatic charge can leak to the earthed paint tank via the usual long and narrow paint supply conduit. Where the paint supply conduit is too lengthy and of a narrow cross-section the amount of paint throughflow is insufficient. The high-tension generators conventionally used in the painting industry cannot even at maximum load, generate sufficient current to flow in the direction of the tank, as a result of the low resistance of the paint. Thus short-circuits occur and the potential falls to zero.
Although it is possible to place the tank on insulated or earthed legs in order to prevent the charge that has leaked into the tank from going to earth, nevertheless even here the tank has a significant potential relative to earth, and consequently so does the spraying apparatus, which circumstance carries the risk of shocks for an operator. At the same time one must ensure that the charge accumulated in the tank should be able to leak or flow away after switching the apparatus on. This can be solved by providing an earthing apparatus or a line connected to the tank.